THE IOWAN DRIFT SHEET. 133 



rather questionable, especially since it calls for a movement north of west. 

 It is, however, not so narrow a tongue as appears to have been thrust west- 

 ward into the Pecatonica Basin, and projects but 4 or 5 miles beyond a regu- 

 lar border in line with that in eastern Whiteside County. The difficulties 

 seem scarcely as great as would be involved in an invasion of ice from 

 eastern Iowa into this district at the Iowan stage of glaciation. There are, 

 however, in northwestern Whiteside and southwestern Carroll counties 

 features which raise the suspicion that ice from the Iowa side may have 

 crossed into that district in the Iowan stage of glaciation and extended as 

 far east as the meridian of Morrison. It becomes necessary, therefore, to 

 consider the question of an extension of the Iowan ice sheet from Iowa 

 southeastward into this tract. This question is considered later (p. 144).- 



It is difficult to determine the position of the border of this ice sheet 

 south from Rock River, since there is a broad area occupying the interval 

 between Rock and Green rivers, in which heavy deposits of sand occur. 

 Exposures of fresh-looking- till north of Spring Hill seem to be of Iowan 

 age. South from the Green River sand deposits there is a narrow tract of 

 low country extending eastward from Geneseo to the vicinity of Sheffield, 

 in which there is very little sand, and also very little loess compared with 

 the covering on the higher districts to the south, and which may prove to 

 have been occupied by the Illinois lobe of the Iowan ice sheet during the 

 loess deposition. Its south border is within 2 or 3 miles south of the Chi- 

 cago, Rock Island and Pacific Railway throughout the interval between 

 Geneseo and Sheffield, and is marked by a bluff-like rise of 40 to 60 feet to 

 a belt of heavy loess. Immediately east of Sheffield the outer moraine of 

 the Wisconsin drift sets in, and no drift attributable to the Iowan invasion has 

 been recognized outside its limits in districts to the south. There appears, 

 therefore, to be striking- similarity between this lobe of Iowan age and that 

 which occupied eastern Iowa. They each show at the north remarkable pro- 

 trusions, extending- in both cases nearly to the borders of the Driftless Area. 

 Their margins also are strikingly different in outline from those of preceding 

 and succeeding sheets in the same districts. The east Iowan sheet has been 

 found to extend only about to the latitude of Rock Island and Muscatine, 

 and possibly the lobe which we are considering extended no farther south. 

 There are, however, till deposits in eastern Illinois, as far south as Iroquois 

 County, which are suspected to be of Iowan age, as indicated below. 



